Is there anything magic about ReadOnlyCollection

Question

Having this code...

var b = new ReadOnlyCollection<int>(new[] { 2, 4, 2, 2 });
b[2] = 3;

I get a compile error at the second line. I would expect a runtime error since ReadOnlyCollection<T> implements IList<T> and the this[T] have a setter in the IList<T> interface.

I've tried to replicate the functionality of ReadOnlyCollection, but removing the setter from this[T] is a compile error.

Answer 1

The indexer is implemented with explicit interface implementation, so you'll only be able to access it if you do:

IList<int> b = new ReadOnlyCollection<int>(new[] { 2, 4, 2, 2 });
b[2] = 3;

or

var b = new ReadOnlyCollection<int>(new[] { 2, 4, 2, 2 });
((IList<int>)b)[2] = 3;

Of course, it'll then fail at execution time...

This is entirely deliberate and helpful - it means that when the compiler knows it's a ReadOnlyCollection, the unsupported bits of functionality aren't available to you, helping to divert you away from execution time failure.

It's an interesting and relatively unusual step though, effectively implementing one half of a property/indexer implicitly, and one half explicitly.

Contrary to my previous thoughts, I believe ReadOnlyCollection<T> actually implements the whole indexer explicitly, but also provides a public readonly indexer. In other words, it's something like this:

T IList<T>.this[int index]
{
    // Delegate interface implementation to "normal" implementation
    get { return this[index]; }
    set { throw new NotSupportedException("Collection is read-only."); }
}

public T this[int index]
{
    get { return ...; }
}